Method and apparatus for recording and reproducing sound-on-film



Feb. 22, 1938. L. DEWAN 2,108,801

METHOD AND APPARATUS FOR RECORDING AND REPRODUCING SOUND-ON-FILM Filed June 16, 1954 Patented Feb. 22, 1938 PATENT orrica METHOD AND APPARATUS FOR RECORDING AND PRODUCING SOUND-ON-FIIM Leon Dewan, New York, N. Y., assignor of fii'ty percenttoGeorgeH.

Callag an, New York.

Application III-he 18, 19;, Serial No. 730,972

This invention relates to improvements in sound recording and reproducing on film, and has for its object the recording of sound photographically by means of an incandescent filament in such a way that the high frequency components of the voice may be recorded upon the tone track of the usual mm. films and also upon tone tracks of more slowly moving films such as the 16 mm. and even an 8 mm. size.

Another object of this invention is to produce a filament lamp for the purpose above mentioned that will be of microscopic dimension and still maintain a reasonably long life.

Another object of this invention is an optical arrangement associated therewith which shall assure satisfactory results in this system by making it possible to collect maximum light from the filament without aberration and by eliminating the halo which usually surrounds the image of a filament upon a film.

The foregoing and other objects in view will appear as the description proceeds.

In this system variable electrical currents are impressed upon a minute incandescent filament of platinum or tungsten which is imaged upon a moving film. As the temperature of the filament varies in accordance with the variable current, its eiIect upon the film varies accordingly.

The filament in this case is very minute and a0 is in the form of a ribbon whose face is presented toward the film. Its width and length may vary according to what type of film is used and whether it is optically reduced or not but the thickness of the ribbon remains practically constant relative to the temperature employed as the speed of temperature response to the variable impressed current depends upon this dimension and the degree of the temperature of the filament. The filament is in ribbon form for the reason that a certain width of the filament is necessary to give sufiicient light emitting surface. Were the ribbon of round cross section of the required diameter its mass would be too great to allow the speed of temperature response necessary.

In order to give the filament reasonably long life, an inert gas under several atmospheres of pressure is introduced within the lamp bulb. This high pressure retards volatilization and makes it possible to use the microscopic filament. In order to quicken the temperature response and make speedy operation possible at a lower temperature than would otherwise be the case the lamp is packed in a refrigerating compound, as for example, solidified carbon dioxide. This allows the filament to respond as quickly to the impressed voice currents as one whose temperature is very much higher and whose bulb is left at room temperature.

In imaging the filament upon the film it is only slightly reduced; optically or not at all. Thus its length is kept small and the filament thereby rendered structurally firm despite its extreme thinness which is a constant quality, independent of any other dimensions which may be given to the filament in accordance with the optical arrangement used.

Figure l is a diagrammatic view of a complete recording system embodying this invention.

Figure 2 shows the efi'ect upon light passing through a plane window.

Figure 3 shows the efiect of a convex window upon the emerging light.

Figure 4 shows the effect of a lens window upon the emerging light.

Figure 5 shows a cross section means for ailowing i'or filament expansion.

Figure 6 shows a method of producing the filament.

Figure 7 shows a circuit usable in this system.

Figure 8 shows graphically a condition of the circuit of Fig. 7.

Figure 9 shows the structural arrangement for the light source, mirror and film.

In Figure l, the filament I is imaged by the concave mirror 2 upon the tone track of the film 3 which is viewed from above. The mirror which may be produced by a bright deposit of aluminum or silver upon a glass base to project a large percentage of the actinic rays, is spherical in form, and as shown by the dotted lines continuing its curve, the filament I and the image on the film are at approximately the center of curvature of the mirror and as near as possible to its principal axis 4-4. As such a mirror is not only simple and inexpensive of construction but spherical and chromatic aberration are avoided as a light wave originating in the plane at the centre of curvature tends to return thereto unchanged,

and therefore the image formed on the film is .1

sharp and clear. This arrangement also makes it possible to take a large part of the sphere of the emitted light without impaired quality.

The result is that the filament is not required M to be maintained at an excessively high temperature as this arrangement makes it possible to use a large part of the light to retain its actinic nature due to the lack of absorption involved The thinness of the ribbon filament rather than an excessively high temperature gives the filament its quick responsiveness.

It will be seen that while the light source I and the film 3 are in the vicinity of the center of curvature of the mirror and as near as possible to the principal axis thereof, they do not lie exactly in the plane of the center of curvature but are slightly tilted relatively thereto so that any point of either the light source or the film is moved away from the plane of the center of curvature toward the center of the mirror as that point is farther from the principal axis. The reason for this is that since the light source and the film are necessarily located to either side of the principal axis andthe projection is therefore eccentric a slight distortion would occur if the light source and the fllm remained exactly in the same plane since the points of the light source would come to a focus nearer to the center of the mirror as these points are farther from the principal axis. Therefore by means of this tilt the 'real image formed of the light source comes nearer to coincidence with the plane of the center of curvature and the film surface is tilted to meet this real image so that each point of the light source comes to a focus at the corresponding point on the film.

In recording sound the filament is only a little longer than the width of the tone track and is ribbon formed to give the necessary width of light emitting area combined with small mass for speedy temperature response. In reproducing however the filament while of approximately the same length and width as the recording filament must not be ribbon formed but must be of solid or cylindrical cross section in order to attain the high temperature necessary to actuate the photocell properly and at the same time maintain reasonably long life.

The ribbon shaped recording filament which may range between .002" and .0001" or less in width depending upon what size of sound track is desired or whether any optical reduction is employed, has a thickness of between one half to one twentyfifth of its width. For example, with 16 mm. film sound track and no optical reduction of the filament, the filament may have a width of .0005 and a thickness of .0001.

Since the speed of response depends upon the high temperature at which the filament is worked as well as its thinness, and since the life of the filament is reduced by an excess of either of these factors, the thickness of the filament is made such that the necessary speed of response is attained at a temperature which does not greatly shorten the life of the lamp.

Where it is desired to optically reduce the image of the filament, the filament may be placed farther away from the concave mirror and a small diverging lens placed at a point between the mirror and its centre of curvature. The distance between mirror, lens and filament and the divergence ,of the lens are such that the lens produces a. reduced virtual image at the plane of the center of curvature of the mirror. The length and the width of the filament would then be increased in accordance with the degree of reduction employed but its thickness would not be made to difier greatly in spite of any changes in width or length since increase in the thickness of this ribbon filament would affect the speed of temperature response to current fluctuation and introduce lag.

Instead of a divergentlens a convex mirror may be used to produce the same effect. The

convex mirror may be placed at a point between the concave mirror and the virtual image produced by the convex mirror in the plane of the center of curvature of the concave mirror.

In Figure l, the lamp 5 is in the form of a small stout tube with a window 6 of thin glass or quartz to permit a large proportion of the actinlc rays to emerge. The interior of the tube is covered with a dull black deposit preferably of carbon paint so that no light except that of the filament itself shall leave the window and the image produced by the concave mirror shall be free of parasitic reflection effects.

The window 6 is not made plane (as has been proposed in the past) but is convex, with its walls forming parts of concentric spheres at whose center is placed the filament I. The reason for this is shown in Figures 2 and 3. The efi'ect of a plane window 1 upon the image on the film 3 produced from a light source I is shown in Figure 2. The internal reflections of the plane window as shown by the dotted lines produces virtual images further back of the plane window and real images ahead of the film 3. One of these images 8 is shown to illustrate. The result is a halo due to the spreading of the light from the secondary real images such as image 8 which falls short of the film.

The effect of the window 6 (Figure 3) with the filament I at its center of curvature is that repeated internal reflections produce real images at the same point as the first real image on the film-3 and virtual images at the same point as the filament I. If the filament I is placed to one side of the center of curvature of the convex window 6 the secondary real image produced by internal reflection of the window will fall to one side of the image on the film and if a mask with an aperture to allow only the first real image to reach the film is placed near the film the image produced by internal reflection will fall upon the mask instead of the tone track.

In Figure 4 is illustrated another method of preventing halation due to internal reflection. The window in this case is in the form of a converging lens 9 and both the window and the filament I are placed nearer to the concave mirror 2 than the film 3. It will be seen that the lens produces a virtualimage at the plane of the center of curvature as shown by the dotted line I0. The internal reflection of the lens causes an image to be produced at X very near tothe con cave mirror and the light therefrom still remain divergent after the reflection from the concave mirror and its effect upon the film I is negligible due to the feebleness produced by the wide dispersion. Repeated internal reflections of the lens produce additional images nearer the lens 9. The light from these images will be more convergent than the first image but their eifect upon the film 3 will also be negligible due to their feebleness caused by repeated reflections.

A mask II having an aperture therein may be used to protect the film 3 from difiused light but the aperture will be larger than the image and will in no way limit its thickness.

The lens 9 should be made of glass or other transparent material having as low a refaction index as possible so that its boundaries may be sufllciently curved to form the reflection images at the points desired without having a great refractive effect upon the issuing light.

In either of the above cases the window need not be round in area but may be roughly oblong follow ng the general form of the filament so a,ioaso1 that the window can be made small to resist the Pressure within the lamp bulb.

In Figure 5 is shown one method of producing the filament. A ribbon I! of metal foil of a thickness equal to the desired width of the filament is mounted upon the supports II and the whole is covered with a protective coating which is scraped away from the edge of the foil and the tops of the supports as shown by the shading. The mounting is then dipped in plating bath and the filament metal such as platinum for example, is deposited on the edge of the ribbon foil and the supports. The protective coating is then removed and the mounting is treated in a solution which dissolves the ribbon foil but leaves the filament exposed.

The filament may also be produced by the Wollaston method by which a filament as fine as .00004 may be drawn. A ribbon shaped wire of the filament metal is covered by another metal and drawn to the desired fineness. The compound wire may then be mounted upon the filament supports, a protective coating applied thereto and to the ends of the compound wire which is then dipped in a solution to remove the outer metal and expose the filament. In mounting the fine filament the tops of the filament supports may each contain a globule of a hot easily fusible metal so that the filament may be applied .thereto and the metal allowed to cool and solidify. A high frequency coil may be used to heat the globules.

In Figure 6 is shown a means of allowing for the expansion of the filament when its temperature is raised. Not only is the filament i bow shaped but a rod ll of insulating material such as glass for example, is heated at the same time as filament I by means of a thick filament i5 which is composed of resistance metal so as not to short circuit the filament i. The rod I4 is placed at such a point between the base of the supports and the filament I that its expansion with temperature rise spreads the supports at their ends where the filament I is located to the exact amount that the'filament I is elongated by the same current. A solid rod of resistance metal may be substituted for the rod H providing that its resistance is made sufiiciently high to prevent excessive conductance. J

In Figure 7 is shown a type of circuit usable in this system. The voice currents of the microphone are amplified and transferred to the circuit of the filament l by means of the transformer i6 so that the effect of the dry cell battery I! on the filament is modified accordingly. When the voice currents are strong the half cycles thereof which oppose thecurrent of the battery I1 not only neutralize it but tend to send currents in a direction against the battery and thereby cause a heating effect where a further cooling is necessary. A rectifier I8 is therefore placed in series as shown with the battery I! so that when a voice current in the transformer secondary exceeds a certain point in opposition to the battery i1 and tends to send current in a reverse direction through the filament I, this current is limited by the rectifier. This would cause the average temperature of the filament I .to rise when the point is reached at which the voice intensity is strong enough to cause the currents in the transformer secondary to exceed the current of the battery ii. In order that this rise in filament temperature shall not reach an excessive point with sound of great intensity, and in order to -make it possible to maintain the filament temperature normally at a point where it will respond sumciently fast to high frequency voice currents, without danger of burning, out when its average temperature rises with great voice intensity; the amplification of the voice current may be limited preferably in the manner'shown in Figure 8 wherein is represented a grid potential-anode current curve. The C"-bias is such that the anode current is at that point of the curve x where a large fluctuation of the grid voltage would cause it to encounter the nonlinear portion of the curve and thereby limit the anode current. It may be so arranged that those half cycles of the voice currents which augment the current of the battery I! are thereby limited.

Referring back to Figure 7, the amplifier I0 is tuned to favor the higher sound frequencies over the low frequencies to a greater extent than is usual in recording to compensate for the reduction in the amplitude of the high frequencies caused by the temperature lag of the filament.

The normal temperature of the filament I when no sound is ailiecting the microphone is such as to produce a photographic eifect on the tone track somewhat below the point of medium tone. Therefore when the sound exceeds the point where the voice currents are stronger than the current of battery I7 and the filament temperature rises as, a result, the fluctuations of the filament brightness will then center about the point of medium photographic effect. To bring the effect of the filament to the point desired in the photographic range a smoked glass filter may be employed or the filament may be made sufficiently fine.

The rectifier It may be of the copperoxide type or may consist of a number of two element vacuum tubes in parallel.

In order to further conserve the life of the filament a negative charge may be applied thereto relative to a conductive coating on the interior surface of the lamp bulb, so that the filament attracts and retains the positive ions. If desired, a separate incandescent filament may be used as an electrode in place of the conductive coating on the interior surface of the lamp bulb. The light from this incandescent filament may be masked so that none of it will issue from the lamp bulb and interfere with the light coming from the regular filament. Opening is left in the mask however so that the discharge from this secondary filament shall reach the regular filament.

In order that the distances between the light source, the concave mirror and the film shall remain constant throughout the recording and that the image shall not lose its sharp definition through small vibrations of the film, the concave mirror 2 in Figure 9 is mounted upon a structure 2! which extends forward from the mirror and terminates in a smooth surface which is pressed somewhat lightly against the film. The lamp 6 is also mounted upon the structure 2|. The part of the structure which presses against the film may be slightly rounded and bear on the film at a point to one side of the sound track or it may be in the form of a window exposing the sound track to the image from the mirror. If desired, the structure 2i, instead of pressing directly against the as shown may extend behind the film to form the axle of a roller over which the film passes. This produces the same effect of keeping the distance between the film and the mirror constant.

While the above invention is shown in connection with sound recording and reproducing, it

of television and the like.

- the variable area sound track is clear and unaffected.

The novel features are claimed broadly and are not bound to the exact wording of the claims.

For example, light source or "linear light source may refer to an aerial image of a filament rather than the filament itself, or for example to light issuing from a slit in a mask upon which light is concentrated.

What is claimed is:-

1. In a sound recording or reproducing system including a light source, a photographic film surface, and a concave mirror of approximately spherical form to project an image of linear form and extreme sharpness in the direction of its width upon the photographic film, the light source and the photographic film surface being placed approximately on a line passing through the center of curvature of the concave mirror at right angles to its principal axis, the focus of said image being constant.

, will be seen that it is also applicable to the field 2. A sound recording or reproducing system including in combination, a linear source of light of great tenuity, a photographic film surface disposed adiacent to the light source, and a concave mirror opposite the light source and the photographic film surface and projecting an image of constant focus of the light source upon the photographic film surface.

3. In a sound recording or reproducing system including a source of light, a photographic film, and a concave mirror for imaging the light source upon the photographic film, the light source and the photographic film being slightly turned from the plane which is normal to the principal axis of the concave mirror so that the image of the light source upon the film is in approximate focus at all points thereof.

4. In a sound recording or reproducing system, a photographic film, means for progressing said film, a fixed concave mirror of generally spherical form, means whereby light is produced and caused to diverge from a plane approximately parallel to the plane of the photographic film, the area from which the light diverges being linear in form, the image on the film being of a highly tenuous linear form and of extreme sharpness in the direction of its width, and the light from the concave mirror converging fixedly to the film to maintain the image focus constantly in the plane of the photographic film surface.

. LEON DEWAN. 

